In an NAND-type non-volatile memory device, reading out the data stored in the selected memory cell is executed by applying a voltage Vread to the control gate of each unselected memory cell and holding the control gate of the selected memory cell at the bias voltage equal to or lower than a threshold value. In some case, however, the bias applied to the unselected memory cells influences the selected memory cell and lowers the threshold voltage thereof. For example, the distance between the memory cells is narrowed in a highly miniaturized memory cell array, making such a threshold voltage drop significant, and deteriorates the read-out performance thereof. Thus, there is a demand for a read-out method capable of suppressing the threshold voltage drop in the selected memory cell.